DCF77 frame parser

dcf.c

#include "dcf.h"
#include <time.h>
#include <inttypes.h>

// 8-bit BCD decoder
static inline int dcf_decode_bcd( uint8_t bits )
{
	return ( bits & 0x0f ) + 10 * ( ( bits >> 4 ) & 0x0f );
}

// Checks parity of `n' bits starting at `start'
static int dcf_parity( uint64_t frame, int start, int n )
{
	int parity = 0;
	uint64_t b;
	for ( b = ( 1ull << start ); b != ( 1ull << ( start + n ) ); b <<= 1 )
		parity ^= !!( frame & b );
	return parity;
}

// Convert DCF77 frame to `struct tm`
static int dcf_parse( struct tm *t, uint64_t frame )
{
	if ( t == NULL ) return 0;

	// Validate frame
	if ( frame & ( 1 << 0 ) ) return 0; // Start of minute - 0
	if ( ~frame & ( 1ul << 20 ) ) return 0; // Start of time - 1
	if ( ~( frame ^ ( frame >> 1 ) ) & ( 1ul << 17 ) ) return 0; // Exclusive CET / CEST
	if ( dcf_parity( frame, 21, 8 ) ) return 0; // Minute parity
	if ( dcf_parity( frame, 29, 7 ) ) return 0; // Hour parity
	if ( dcf_parity( frame, 36, 23 ) ) return 0; // Date parity
	
	// Construct time struct
	t->tm_sec = 0;
	t->tm_min = dcf_decode_bcd( ( frame >> 21 ) & 0x7f ); // 7 bits
	t->tm_hour = dcf_decode_bcd( ( frame >> 29 ) & 0x3f ); // 6 bits
	t->tm_mday = dcf_decode_bcd( ( frame >> 36 ) & 0x3f ); // 6 bits
	t->tm_mon = dcf_decode_bcd( ( frame >> 45 ) & 0x1f ) - 1; // 5 bits
	t->tm_year = dcf_decode_bcd( ( frame >> 50 ) & 0xff ) + 100; // 8 bits
	t->tm_wday = dcf_decode_bcd( ( frame >> 42 ) & 0x07 ) - 1; // 3 bits
	t->tm_yday = 0; // FIXME?
	t->tm_isdst = !!( frame & ( 1ul << 17 ) ); // CEST
	
	return 1;
}

// Enqueues an incoming data bit for parsing
int dcf_pushbit( struct tm *t, int state, int duration )
{
	static uint64_t buffer = 0; // Frame buffer
	static int length = 0; // Number of bits received so far
	int bit = 0; // Received bit value. Ignore if `err' is set
	int err = 0; // Set if impulse has invalid duration
	
	// Determine bit value
	if ( state == 0 )
	{
		if ( duration < 600 || duration > 1400 ) err = 1;
	}
	else
	{
		if ( duration >= 40 && duration <= 130 ) bit = 0; // 0 - 100ms
		else if ( duration >= 140 && duration <= 250 ) bit = 1; // 1 - 200ms
		else err = 1;
	}
	
	// Check bit error
	if ( err )
	{
		if ( length == 59 )
		{
			err = dcf_parse( t, buffer );
			buffer = 0;
			length = 0;
			return err;
		}
		else
		{		
			buffer = 0;
			length = 0;
			return 0;
		}
	}
	else if ( state == 1 )
	{
		buffer >>= 1;
		buffer |= ( (uint64_t) bit << 58 );
		length++;
	}
	
	return 0;
}

dcf.h

#ifndef DCF_H
#define DCF_H

#include <time.h>

/**
	\brief Enqueues incoming data bit from DCF77 for parsing
	\param t Pointer to `tm` struct that shall be written upon succesful parsing
	\param state The input state (high/low)
	\param duration The duration of the input state
	\returns A non-zero value if a meaningful frame was parsed
*/
extern int dcf_pushbit( struct tm *t, int state, int duration );

#endif